1. Field of the Invention
The present invention relates generally to organic light-emitting devices, and to organic light-emitting compounds for use in such devices. In particular, the invention relates to organic light-emitting devices incorporating dibenzoquinoxaline derivatives.
2. Related Background Art
Electro-luminescent and electro-phosphorescent organic light-emitting devices (xe2x80x9cOLEDsxe2x80x9d) are promising for flat-panel display applications. Successful commercialization of such flat-panel displays depends improvements in certain performance parameters, such as, e.g., efficiency, life-time, and color purity. Such improvements depend, in part, on the development of novel light-emitting materials. As OLEDs work through a double charge injection from two electrodes, i.e., holes are injected from the anode into the highest occupied molecular orbital (HOMO) of the hole transport layer molecules, and electrons from the cathode into the lowest unoccupied molecular orbital (LUMO) of the electron transport layer molecules, an improvement in the ability to inject charge in OLED materials has been an important approach in device development.
Essentially, a typical OLED comprises a transparent electrode of indium-titanium-oxide (xe2x80x9cITOxe2x80x9d) or other transparent electrode material formed on a glass substrate, an organic amine based hole transporting layer laminated onto the transparent electrode, an organic light emitting layer formed of a material having electronic conductivity and giving out strong light emission, e.g., tris(8-quinolinolato)aluminum complex (xe2x80x9cAlQ3xe2x80x9d), and an electrode provided on the organic light emitting layer, formed from a material having a low work function, such as, e.g., an MgAg material. Typically, at least one organic compound layers is interleaved between a hole injecting electrode and an electron injecting electrode, and the organic compound layer has a double- or triple-layer structure.
In double-layer structures, a hole transporting layer and a light emitting layer are formed between the hole injecting electrode and the electron injecting electrode, or a light emitting layer and an electron transporting layer are formed between the hole injecting electrode and the electron injecting electrode. In triple-layer structures, a hole transporting layer, a light emitting layer and an electron transporting layer are provided between the hole injecting electrode and the electron injecting electrode.
In general, organic materials used in OLEDs have a higher barrier to electron injection at the cathode-organic layer interface and a lower electron mobility than do inorganic materials used in such devices. As a result, a charge injection imbalance between the holes and electrons and the charge density near the interface of the two organic layers may occur. Therefore, an improvement in electron injection ability, and the employment of an electron transporting layer in multi-layered devices can provide improved device work efficiency and lifetime. The luminescent properties and electron injection ability of AlQ3 are both good, and, thus, AlQ3 has been utilized in various high efficient OLED devices. However, as the peak emission of AlQ3 is 526 nm, i.e., in the green portion of the visible spectrum, its application in blue and red devices is limited. As a result, wider band gap materials would be more desirable than AlQ3, and a need exists for electron-transporting materials having a band gap covering the entire visible spectrum from purple to red.
U.S. Pat. No. 6,303,239 to Arai, et al. discloses an organic electro-luminescent device, comprising a substrate, a pair of a hole injecting electrode and a cathode formed on said substrate, and an organic layer located between these electrodes, which, at least in part, performs the light emission function. An inorganic insulating electron injecting and transporting layer is provided between the organic layer and the cathode. The inorganic insulating electron injecting and transporting layer comprises a first component comprising at least one oxide selected from the group consisting of lithium oxide, rubidium oxide, potassium oxide, sodium oxide and cesium oxide, a second component comprising at least one oxide selected from the group consisting of strontium oxide, magnesium oxide and calcium oxide, and a third component comprising silicon oxide and/or germanium oxide. The disclosed light emitting layer contains a fluorescent material in combination with a host substance capable of emitting light by itself; i.e., the fluorescent compound is used as a dopant. Quinolinolato complexes, and aluminum complexes containing 8-quinolinol or its derivatives as ligands are disclosed as the host substance.
U.S. Pat. No. 6,277,503 to Hashimoto, et al. discloses an organic electroluminescent component, comprising an aromatic methylidene compound of the general formula
Arxe2x80x94[xe2x80x94CHxe2x95x90CHxe2x80x94Arxe2x80x94CHxe2x95x90C(Ar)R]n
where the various Ar groups are substituted or unsubstituted aromatic groups, and n is 1 to 6. The construction of the organic electroluminescent component is typically a luminescent layer interposed between a pair of electrodes, i.e., a cathode and an anode, and may further comprise a positive hole-transporting layer and an electron-transporting layer.
U.S. Pat. No. 5,861,219 to Thompson, et al. discloses organic light emitting devices containing a metal complex of 5-hydroxy-quinoxaline as a host material. The disclosed organic light emitting devices have an electroluminescent layer containing a host material comprising a metal complex of (5-hydroxy)quinoxaline and a dopant material comprising at least one of a bisphenyl-squarilium compound, an indigo dye compound and a fullerene compound.
U.S. Pat. Nos. 6,245,449 and 6,150,042 to Tamano, et al. disclose a hole-injecting material, having a triphenylene structure, for an organic electro-luminescent device and an organic electro-luminescent device using the disclosed hole-injecting material. The disclosed organic electroluminescence device comprises a light-emitting layer or a plurality of organic compound thin layers including the light-emitting layer between a pair of electrodes composed of a cathode and an anode, where at least one layer contains the disclosed hole-injecting material.
U.S. Pat. No. 6,255,449 to Woo, et al. discloses 2,7-dihalofluorenes, substituted at the 9-position, methods for the preparation of such 9-substituted-2,7-dihalofluorenes, oligomers and polymers of the disclosed fluorine compounds, films and coatings prepared from such florins, oligomers and polymers, processes for preparing such films and coatings, and light-emitting diodes comprising one or more layers of the polymer films, where at least one of the layers is derived from the oligomers and polymers of the invention.
U.S. Pat. No. 6,268,072 to Zheng, et al. discloses an electroluminescent device comprising an anode, a cathode, and polymer luminescent materials disposed between the anode and cathode, where the disclosed polymeric luminescent materials are 9-(4-adamantanyl)phenyl)-10-phenylanthracene-based polymers.
U.S. Pat. No. 5,792,567 to Kido, et al. discloses organic electroluminescent devices having a layer comprising at least one triazole derivative. Preferably, the layer is an electron-transport layer containing the oligomerized triazole derivatives as an electron-transport material.
U.S. Pat. No. 5,904,994 to Dodabalapur, et al. discloses blue emitting materials, which, when placed in electro-luminescent devices with an emitter layer and a hole transporter layer, provide white light. The disclosed materials are non-polymerizable. i.e., containing no ethylenic unsaturation, and contain at least one unsaturated, five or six-membered heterocyclic moiety with at least one nitrogen atom incorporated into the heterocycle. These moieties are incorporated into molecules that form amorphous films when deposited using conventional techniques such as vacuum sublimation. The moieties are selected from the group consisting of oxazole, imidazole, quinoline, and pyrazine.
There is no known prior art disclosure of organic light-emitting devices utilizing the advantageous properties of the dibenzoquinoxaline derivatives discussed below. The present invention provides such organic light-emitting devices.
The present invention is directed to electron transporting materials and organic light emitting devices comprising the electron transporting materials, wherein the electron transporting materials are dibenzoquinoxaline compounds, and the devices comprise at least one layer comprising at least one dibenzoquinoxaline compound. Dibenzoquinoxaline compounds of the invention include dipyridyl substituted dibenzoquinoxaline compounds, diphenyl substituted dibenzoquinoxaline compounds, naphthalene substituted dibenzoquinoxaline compounds, anthrene-dibeanzoquinoxaline compounds, bis(methoxy)phenyl substituted dibenzoquinoxaline compounds, dithiophen-dibenzoquinoxaline compounds, and dibenzoquinoxaline compounds. Preferably, the at least one dibenzoquinoxaline compound is present in the electron transporting material or the at least one layer in an amount of at least about 1 percent by weight.